Document MMIO behaviour

[danielsn]

Currently, there is no documentation for how mmio works and what its semantics is. Anyone using mmio will need documentation.

[jimgrundy]

Why is this medium @danielsn now that the allocated_memory is deprecated?

[jimgrundy]

I'm dropping "medium" as I don't know of a driving use case right now. Feel free to restore if you do @danielsn

[markrtuttle]

For the record, there is some description of the MMIO functionality in the CAV paper that added it: https://link.springer.com/content/pdf/10.1007/s10703-020-00344-2.pdf